Did you make this instructable?

Here's a tutorial on how to change skin texture/color using Photoshop. The process would be very similar in GIMP (although more frustrating). The author of that how-to rated it as beginner, but I wouldn't call it that. It is a somewhat difficult technique but totally do-able so long as you're willing to spend an hour or two on it to make it look natural.

Depending on how far you want to go, you'll also need to widen the bridge of the nose, spread the eyes further apart, increase the size of the eyes, change the eye color, replace the ears, and potentially change the hair color. To make it as "easy" as possible, do a straight on head shot against a plain backdrop.

I'd recommend just changing the skin color/texture, eye color, ears, and hair color if necessary. When you start rearranging facial features it gets a lot more tricky.

I'd say that's a good answer. The process would be about the same in most photo editors, since they all have roughly the same functions and share most of the same heritage.

Personally, I'd start by getting as close as I could with costume and makeup, then use software to take it the rest of the way. Then again, I know folks who could probably construct a build-up costume which would let them believably walk out on stage as a 9-foot-tall blue alien with prehensile tail. (http://www.costume.org/ -- I'm officially a Journeyman, but only because I happened to be drafted to help a Master costumer present a winning contest entry.)